7.
Key Concepts
Polychlorinated biphenyls (PCBs): a mixture of
compounds containing the biphenyl structure with
varying numbers (i.e., one to ten) and arrangements of
chlorine atoms attached.
Aroclor: One of nine commercial PCB mixtures, with
varying levels of chlorination, formerly produced in the
United States. The percent of chlorine content varies
across the different Aroclors, depending on their
intended uses. Generally, samples of pure Aroclor
exhibit a distinct pattern of congener concentrations, but
transformations during routine use and/or weathering can
alter these patterns.

8.
Key Concepts
Congener: One of the 209 possible PCB molecules, each
distinguished by the number and arrangement of chlorine
atoms. Commercially-produced PCB mixtures collectively
include about 175 congeners, some at concentrations so
low they are not detectable in environmental samples.
One hundred and ten of the 209 congeners typically
constitute 98 percent of PCB mass measured in samples.
Dioxin-Like Congener: One of 12 PCB congeners that
exhibits toxicity similar to that of dioxin as the result of
structural similarity to 2,3,7,8-tetrachlorodibenzo-p-dioxin
(2,3,7,8-TCDD). The 12 dioxin-like PCB congeners are:
77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169 and
189.

9.
Key Concepts
Dioxins and Furans: Two families of chemicals related
by their similar physical and biological characteristics.
Several hundred different compounds exist among the
chlorinated dibenzo-p-dioxins (CDDs) and the
chlorinated dibenzofurans (CDFs). The term “dioxin” is
often used to refer to the most toxic dioxin compound,
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). CDDs and
CDFs are products of combustion created by
anthropogenic activities and natural processes.
Weathering: Physical, biological, or chemical processes
that can alter the chlorination pattern of a PCB molecule,
and thereby change the congener composition of
environmental mixtures of PCBs.